Early designs of ring laser gyroscopes comprised a discharge tube filled with a gas containing a lasant and having Brewster angle windows for the emission of polarised laser beams. The discharge tube was located between two mirrors and one or more further mirrors were positioned so that, together, the mirrors defined a polygonal path for two laser beams travelling in opposite directions in a closed loop.
Designs of ring laser gyroscopes have progressed to fabricating these from a single block of dielectric material having a low coefficient of thermal expansion e.g. Zerodur. Such ring laser gyroscopes have the advantage of being sufficiently rugged that they can be dithered to overcome the well-known problem of lock-in of the laser beams. Bores are drilled in the monoblock, as it is known, to define each limb of the lasing cavity as well as extra bores leading to electrodes to enable DC excitation of the gas in the lasing cavity, gas reservoir bores and a gas filling tube. Consequently, precise machining is necessary and the gyroscope mirrors have to form gas-tight seals which means that they need to be relatively large. Moreover, machining of the surfaces for mounting the mirrors needs to be very precise.